CHOOSING A CCD

The main difference between CCD cameras is the size of their chips. The lowest-priced cameras use chips with a 196 x 165 pixel array, while top-end cameras have arrays of 1,000 x 1,000 pixels or more.

Chips are getting bigger and cheaper each year. The advantage ot a larger chip is that it can record a larger area of sky. This is critical since even large chips are much smaller than a 35 mm film frame, and at focal lengths of 2,000 mm or more, they "see" an area of sky only a few arcminutes wide, smaller than the disk of the Moon. Nevertheless, CCD cameras are superb devices for record-ing planets, galaxies, and smaller nebulas and clusters.

It is important to match the CCD camera to a telescope of the appropriate focal length. Many of today's CCD cameras have tiny pixels just 9 microns, or 0.009 mm, across. Attached to a telescope with a focal length of at least 1,000 mm, these high-resolution cameras will produce images as sharp as the atmosphere allows. When used on shorter focal length telescopes, however, the pixels will be larger than the star images, yielding odd-looking square stars.

Most CCD cameras are black and white. To produce a color image, you need to take three exposures, one each through red, green, and blue filters, and then merge them in the computer using image-processing software.

Producing a clean, noise-free image requires taking two calibration frames. A "dark frame"—an image with the same exposure time as the main exposure but taken with the lens cap on—records the background noise inherent in all cameras. A "flat field" frame—a short exposure of a blank screen or bright light— records flaws such as dead pixels and dust specks. You can then use image-processing software to electronically subtract these unwanted defects from the main image.

CCDs are not devices for the technologically challenged, but if you love computers, you will love CCD imaging.

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